Proposed course for the next fiscal year follows: (1) similar studies will be performed to determine the control mechanisms of xenotropic viruses from other species; (2) xenotropic strains isolated from FMR virus stocks will be studied to determine whether they represent recombinants between FMR virus and the endogenous xenotropic virus present in the cells they are replicating in; (3) human x mouse S plus L minus cells will be used to develop assays for primate tropic viruses; (4) cell lines with biochemical markers will be tested for their usefulness in generating and isolating hybrid cells; (5) S plus L minus SC-1 cells will be isolated for potential use as assay cells for all mouse ecotropic viruses.